Ultrasensitive Self-Driven Terahertz Photodetectors Based on Low-Energy Type-II Dirac Fermions and Related Van der Waals Heterojunctions

The exotic electronic properties of topological semimetals (TSs) have opened new pathways for innovative photonic and optoelectronic devices, especially in the highly pursuit terahertz (THz) band. However, in most cases Dirac fermions lay far above or below the Fermi level, thus hindering their successful exploitation for the low-energy photonics. Here, low-energy type-II Dirac fermions in kitkaite (NiTeSe) for ultrasensitive THz detection through metal-topological semimetal-metal heterostructures are exploited. Furthermore, a heterostructure combining two Dirac materials, namely, graphene and NiTeSe, is implemented for a novel photodetector exhibiting a responsivity as high as 1.22 A W⁻¹, with a response time of 0.6 µs, a noise-equivalent power of 18 pW Hz^−0.5, with outstanding stability in the ambient conditions. This work brings to fruition of Dirac fermiology in THz technology, enabling self-powered, low-power, room-temperature, and ultrafast THz detection.     

Tunneling Conductance in Strained Graphene-Based Superconductor: Effect of Asymmetric Weyl–Dirac Fermions

Based on the BTK theory, we investigate the tunneling conductance in uniaxially strained graphene-based normal metal (NG)/barrier (I)/superconductor (SG) junctions. In the present model, we assume that by depositing the conventional superconductor on the top of the uniaxially strained graphene, normal graphene may turn to superconducting graphene with the Cooper pairs formed by the asymmetric Weyl–Dirac electrons, the massless fermions with direction-dependent velocity. The highly asymmetrical velocity, v _y /v _x ≫1, may be created by strain in the zigzag direction near the transition point between gapless and gapped graphene. In the case of highly asymmetrical velocity, we find that the Andreev reflection strongly depends on the direction of strain, and the current perpendicular to the direction of strain can flow through the junction as if there were no barrier. Also, the current parallel to the direction of strain anomalously oscillates as a function of the gate voltage with very high frequency. Our predicted result is quite different from the feature of the quasiparticle tunneling in the unstrained graphene-based NG/I/SG conventional junction. This is because of the presence of the direction-dependent-velocity quasiparticles in the highly strained graphene system.     

Quantum Phase Transitions and Superconductivity in Single- and Two-Layer Cuprates in the Multiband Theory of Hubbard Fermions

We consider the doping dependence of the normal and superconducting properties of La₂Sr_2?x CuO₂ and YBa₂Cu₃O₇ in the low energy effective model based on the ab initio LDA+GTB calculations. With doping we have found a concentration region with electronic instability of the uniform state. We have shown that two quantum phase transitions (QPT) of the Lifshitz type correspond well to the experimental phase diagram. For superconducting state we have considered both magnetic and phonon mechanisms of pairing.     

Single-layer and bilayer graphene superlattices: collimation, additional Dirac points and Dirac lines

We review the energy spectrum and transport properties of several types of one-dimensional superlattices (SLs) on single-layer and bilayer graphene. In single-layer graphene, for certain SL parameters an electron beam incident on an SL is highly collimated. On the other hand, there are extra Dirac points generated for other SL parameters. Using rectangular barriers allows us to find analytical expressions for the location of new Dirac points in the spectrum and for the renormalization of the electron velocities. The influence of these extra Dirac points on the conductivity is investigated. In the limit of δ-function barriers, the transmission T through and conductance G of a finite number of barriers as well as the energy spectra of SLs are periodic functions of the dimensionless strength P of the barriers, Pδ(x) = V(x)/?v(F), with v(F) the Fermi velocity. For a Kronig-Penney SL with alternating sign of the height of the barriers, the Dirac point becomes a Dirac line for P = π/2+nπ with n an integer. In bilayer graphene, with an appropriate bias applied to the barriers and wells, we show that several new types of SLs are produced and two of them are similar to type I and type II semiconductor SLs. Similar to single-layer graphene SLs, extra 'Dirac' points are found in bilayer graphene SLs. Non-ballistic transport is also considered.     

Dirty bosons: Screening,inert layers and the Lifshitz tail

The stability of the condensate in the presence of a weakly random substrate is shown to be due to screening of the random potential by distortions of the condensate. However the depletion of the condensate and the speed of sound are both affected: increased and decreased respectively. In the case of strong potential fluctuations He atoms may be trapped (remnants of single-particle localisation in the form of Lifshitz tail states). The circumstances under which this gives a picture of the inert layer will be discussed, and a surprising relation to dissipative tunneling will be mentioned.     

基于四次方程的三角解研究非球对称动态黑洞的"类视界面"

给出了具有实用价值的四方程的三角解正根公式，并以匀加速直线运动的Kerr黑洞为例，成功的研究了非球对称动态黑洞的“类视界面”。     

Transitions between crazing,fracture and yield under hydrostatic pressure

A simple model is proposed to explain transitions from crazing or fracture to shear yielding and vice-versa with increasing superposed pressure. The model is based on an estimate of the local tensile component of stress in the vicinity of a flaw, distinguishing between the different physical situations which arise depending on whether or not the pressurizing medium penetrates the flaw. The model explains naturally many observations of specimens failing in a brittle manner in nominally compressive stress fields.     

Raman Spectroscopy, Structural Modifications, and Phase Transitions in the High-Temperature Superconductors

The formation of phases is investigated from a systematic study of the Raman active phonons in the whole range of oxygen doping of YBa₂Cu₃O _x (6 < x < 7). Changes in the spectral characteristics of the in-phase vibrations of the plane oxygen atoms in the overdoped region are related, with slight modifications of the buckling in the CuO₂ planes. A softening of the A _g symmetry apical oxygen phonon is accompanied with an increase in the intensity of the high-frequency mode at 600 cm^–1 and attributed to the breaking of the chains into small segments. The analysis of the apex and the in-phase phonon modes indicates the formation of superstructures, which are embedded as spatially separated phases in the whole oxygen concentration region of 6.35 < x < 6.95. A similar separation into small domains of phases may also occur in the underdoped compounds (x < 6.35) down to a critical oxygen concentration at x 6.15, where the system breaks into randomly oriented small chains.     

Work, Free Energy and Dissipation in Voltage Driven Single-Electron Transitions

We apply the general procedure presented by Jarzynski (C.R. Phys. 8:495, 2007) to the problem of dissipation in a voltage-driven single-electron box. We obtain the expression of dissipated work, and find its relation to the dissipation Q obtained in Averin and Pekola (Europhys. Lett. 96:67004, 2011). We show that the two quantities are identical in common gate protocols where the system makes a transition for sure.     

Transitions of electrons and holes drive diffusion in crystals,glasses and melts

Diffusion of atoms or molecules (generally: particles) is driven by differences and gradients of the chemical potential of the particles in their accessible space. If the difference of the chemical potential is due to differences of concentrations alone, one arrives at the diffusion equations of Fick. The diffusion coefficients are described in known models by vibrations of atoms in condensed matter which cause the exchange of preferentially neutral particles with neighbouring particles, impurities, interstitial places and vacancies near or on surfaces, grain boundaries, dislocation lines and in the homogeneous bulk. The rates of electronic transitions, however, increase also in melts and solids of chemically bonded particles with increasing temperature. Such transitions cause large fluctuating deviations of the local energy, the charge distribution and the local chemical and electrical potentials. The fluctuating deviations interact with the core ions and drive particles to interchange. This mechanism that supplements the known mechanisms of diffusion has not yet found adequate attention in the literature until now. Foundations, experimental results, evidence and consequences for diffusion are discussed.     

Potential 2D Materials with Phase Transitions: Structure,Synthesis, and Device Applications

Layered materials with phase transitions, such as charge density wave (CDW) and magnetic and dipole ordering, have potential to be exfoliated into monolayers and few‐layers and then become a large and important subfamily of two‐dimensional (2D) materials. Benefitting from enriched physical properties from the collective interactions, long‐range ordering, and related phase transitions, as well as the atomic thickness yet having nondangling bonds on the surface, 2D phase‐transition materials have vast potential for use in new‐concept and functional devices. Here, potential 2D phase‐transition materials with CDWs and magnetic and dipole ordering, including transition metal dichalcogenides, transition metal halides, metal thio/selenophosphates, chromium silicon/germanium tellurides, and more, are introduced. The structures and experimental phase‐transition properties are summarized for the bulk materials and some of the obtained monolayers. In addition, recent experimental progress on the synthesis and measurement of monolayers, such as 1T‐TaS₂, CrI₃, and Cr₂Ge₂Te₆, is reviewed.     

Magnetic Properties and Phase Transitions in a Diluted Ferromagnet: Ising,XY, and Heisenberg Models

The magnetic properties and phase transition of a ferromagnetic spin-S disordered diluted thin film with a face-centred cubic lattice are investigated using the high-temperature series expansion technique extrapolated with Padé approximant method for Heisenberg, XY and Ising models. The reduced critical temperature of the system t_c=\frack_BT_c2S(S+1)J_b\tau_{\mathrm{c}}=\frac{k_{\mathrm{B}}T_{\mathrm{c}}}{2S(S+1)J_{\mathrm{b}}} is studied as the function of the thickness of the thin film and the exchange interactions in the bulk, and within the surfaces J _b,J _s and J _⊥, respectively. It is found that τ _c increases with the exchange interactions of surface. The magnetic phase diagrams (τ _c versus the dilution x) and the percolation threshold are obtained. The shifts of the critical temperatures T _c(L) from the bulk value (\fracT_c(￥)T_c(L)-1)(\frac{T_{\mathrm{c}}(\infty )}{T_{\mathrm{c}}(L)}-1) can be described by a power law L ^−λ , where λ is the inverse of the correlation length exponent.     

The Landau and Lifshitz hydrostatic "pressure" p(ρ,T) exemplified by thought experiments

The density-related, Landau and Lifshitz hydrostatic "pressure" p(ρ,T), appearing in the modern literature on the stresses in polarizable fluids, and known also as the Abraham-Becker hydrostatic "pressure", is examined in a thought experiment. Fluid in a rectangular channel is put through a cyclic magnetization and dilatation process at constant temperature. A very direct solution for p(ρ,T) is obtained using the concepts of lumped-parameter electromechanics. The properties of this "pressure" as a carrier of density information and as a contributor to the stresses are illustrated.     

Additive Manufacturing of Nanostructures That Are Delicate,Complex, and Smaller than Ever

Additive manufacturing with two‐photon polymerization (TPP) has opened new opportunities for the rapid fabrication of 3D structures with sub‐micrometer resolution, but there are still many fabrication constraints associated with this technique. This study details a postprocessing method utilizing oxygen‐plasma etching to increase the capabilities of TPP. Underutilized precision in the typical fabrication process allows this subtractive technique to dramatically reduce the minimum achievable feature size. Moreover, since the postprocessing occurs in a dry environment, high aspect ratio features that cannot survive the typical fabrication route can also be achieved. Finally, it is shown that the technique also provides a pathway to realize structures that otherwise are too delicate to be fabricated with TPP, as it enables to introduce temporary support material that can be removed with the plasma. As such, the proposed approach grants access to a massively expanded design domain, providing new capabilities that are long sought in many fields, including optics, biology, robotics, and solid mechanics.     

All optical interface for parallel, remote, and spatiotemporal control of neuronal activity

A key technical barrier to furthering our understanding of complex neural networks has been the lack of tools for the simultaneous spatiotemporal control and detection of activity in a large number of neurons. Here, we report an all-optical system for achieving this kind of parallel and selective control and detection. We do this by delivering spatiotemporally complex optical stimuli through a digital micromirror spatiotemporal light modulator to cells expressing the light-activated ionotropic glutamate receptor (LiGluR), which have been labeled with a calcium dye to provide a fluorescent report of activity. Reliable and accurate spatiotemporal stimulation was obtained on HEK293 cells and cultured rat hippocampal neurons. This technique should be adaptable to in vivo applications and could serve as an optical interface for communicating with complex neural circuits.     

Pressure Effects on Superconducting and Magnetic Transitions in Layered Sodium Cobalt Oxides, NaxCoO2

Magnetoresistance for field close to the parallel to the superconducting planes direction has been studied in organic superconductor κ-(BEDT-TTF)₂Cu[N(CN)₂]Br. The results show a first order phase transition between the superconducting state and the normal state at the upper critical field H_c2. The 1^st order transition is evidenced by the hysteretic behavior in the magnetoresistance as a function of field or angle between the field and the plane. A possible model is discussed.     

U－23at％Zr合金的焓、比热容和相变潜热

用下落式铜卡计法测量了室温到１０７３Ｋ之间Ｕ－２３ａｔ％Ｚｒ（即Ｕ－１０ｗｔ％Ｚｒ）合金的焓，计算了８８０Ｋ以下相应的平均比热容和微分比热容。在焓－温度曲线上观察到在８８９、９３４、９６１Ｋ左右焓值的跳跃，这是和Ｕ－Ｚｒ系相图上固相线下前三个相变温度相对应的。算得相应的相变潜热分别是５７９、１１８和２８１２Ｊ／ｍｏｌ。